There are many circumstances in which it may be required to transmit image data captured by a camera to a host computer system for display thereby. For example, web cameras are well known which capture images and transmit image data to a remote host computer for display thereby. It is also common for a host device having camera functions to also have a display for viewfinding and for reviewing images captured by the camera.
In general, the resolution of the display is substantially lower than the resolution of the images by the camera function. As technology advances, the resolution of modem cameras is increasing and the screen on which images are required to be displayed is often necessarily relatively small and has a comparatively low resolution. Further, in many cases, the computation required to receive all of the image data representing a captured image, demosaic the image data (i.e. perform interpolation of all three base colours, namely red, green and blue or cyan, magenta and yellow for each pixel of an image) and reduce it so that the image fits the screen on which it is required to be displayed is too great to be able to effectively provide the viewfinder function referred to above without introducing an unacceptable level of displayed image latency, additional cost and/or greater power consumption.
Thus, the present invention is concerned with ensuring that a display can support the whole of a captured image without the need for high computational complexity or viewfinder latency. In other words, the present invention is related to the minimisation of the amount of data required to be transmitted to the host device and the amount of processing required to be performed by the camera and the host device.
In one prior solution, an image sensor is designed to output images at either full resolution or at a reduced resolution (achieved by, for example, only outputting odd or even columns of pixels). The reduced resolution images can be used to provide the viewfinding function referred to above. However, this solution increases the cost and complexity of the image sensor and, in any event, does not solve the problems outlined above unless the reduced resolution image produced by the sensor matches the viewfinder display resolution for a particular device.
In another prior solution, an additional image processing chip is employed to pre-process image data produced by the image sensor such that it can be scaled to one of many levels or by an arbitrary scale factor ready for display on a screen. However, the use of the additional pre-processing chip can reduce the battery life of a portable device (because of its significant power consumption) and, in any event, substantially increases the overall product cost, which is clearly undesirable.
In yet another prior solution, whereby the camera function is provided in a portable device such as a personal digital assistant (PDA), a software scaling function is applied to an image prior to display thereof This provides a relatively low-cost solution to the problems outlined above, but requires the reading and processing of large amounts of data which, unless the device has a powerful processor and fast data transfer, may introduce significant latency in the viewfinder.
The present invention is of particular importance to imaging devices having a large number, for example, several millions, of picture elements (pixels), which are used, for example, for digital photography. For direct image display, for example on a LCD screen in the viewfinder of the camera or on a host computing system, it is often desirable to use a memory matrix in addition to the imaging matrix. As explained above, in view of the large number of pixels involved, it is not possible to display all pixels in the viewfinder during operation. For this reason, it is not generally necessary to provide the device with a memory matrix having an equally large number of storage sites as the imaging matrix. A much smaller memory is sufficient, so that the dimensions of the device can be kept within reasonable limits and, since the minimum memory required for display purposes may not hold all of the charge packets generated in the imaging matrix, only a part of the captured image data is selected and stored in the memory matrix, while the rest of the data is disregarded. This is often referred to as “sub-sampling”.
International Patent Application No. WO99/62245 describes an arrangement in which only a part of selected rows of charge packets are read out and the rest of the rows are discarded, wherein, if a first row is read, then at least to succeeding rows are not read. However, this technique specifically relies on interlacing and specifically avoids reading out information from localised groups of pixels, with the result that significant image data may be lost and the quality of the resultant displayed image is compromised.
We have now devised an arrangement which overcomes the problems outlined above.